Poly(tetramethylene ether) glycol is widely used as the polyol component with isocyanates to make thermoplastic polyurethanes. In this application, the poly(tetramethylene ether) glycol imparts both flexibility and hydrolytic stability to polyurethane products.
Poly(tetramethylene ether) glycol, which for convenience will be referred to hereinafter as PTMEG, is made by polymerizing anhydrous tetrahydrofuran (THF) in the presence of an acid catalyst such as fluosulfonic acid. When using an acid catalyst, the polymer is converted in part to esters which are hydrolyzed with water to obtain higher, more economic yields of the polyol product. In U.S. Pat. No. 2,751,419 to Hill and Schulze, it is disclosed that this may be accomplished by "drowning" the polymerization system with hot water. Unreacted tetrahydrofuran is then removed from the resultant aqueous dispersion by conventional stream stripping.
The acidic aqueous dispersion of impure PTMEG is then subjected to washing with water. The washed PTMEG product dispersion is neutralized with lime, decanted to remove most of the water and solids, which are recirculated to the washing operation. The product PTMEG is recovered by drying and filtering.
A substantial amount of aqueous acidic effluent results from the PTMEG washing. Because this stream is acidic and also because it contains small amounts of organics, disposal of this stream becomes a problem; and furthermore, the yield of PTMEG is substantially reduced by losses of product in the effluent from the washing step. For example, the yield loss from this source alone will be on the order of about 2% for relatively high molecular weight grades, but rises to above 10% for the lower molecular weight grades. Thus, from the standpoint of effluent treatment, it would be desirable to lower the chemical oxidation demand (COD) to the effluent stream. Furthermore, from the standpoint of process economics, it would be desirable to reduce the amount of materials wasted whether as raw material or as PTMEG product.